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In the past, builders rarely thought about ventilation for the houses they built. Standard practice resulted in houses that kept out big, uncomfortable drafts, but that still allowed in plenty of fresh air. Today, builders are routinely producing houses that are much tighter — often without even realizing it (see illustration, next page). It is clear from recent research that builders can no longer rely on air leakage alone to provide enough ventilation. Evidence of lower ventilation rates has been seen in numerous studies done across the U.S. and Canada. A study of 64 randomly selected new houses built in 1984 in Minnesota showed that 80% failed to meet recognized minimum ventilation